Communication devices such as wireless devices may be also known as e.g. user equipments, mobile terminals, wireless terminals and/or mobile stations. A wireless device is enabled to communicate wirelessly in a cellular communications network, wireless communications system, or radio communications system, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
The wireless device may further be referred to as a mobile telephone, cellular telephone, laptop, Personal Digital Assistant (PDA), tablet computer, surf plate, just to mention some further examples. The wireless device in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile device, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or PICO base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. By the base station serving a cell is meant that the radio coverage is provided such that one or more wireless devices located in the geographical area where the radio coverage is provided may be served by the base station. One base station may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless device within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile). In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for wireless devices. High Speed Packet Access (HSPA) is an amalgamation of two mobile telephony protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), defined by 3GPP, that extends and improves the performance of existing 3rd generation mobile telecommunication networks utilizing the WCDMA. Moreover, the 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies, for example into evolved UTRAN (E-UTRAN) used in LTE.
In the context of this disclosure, the expression downlink (DL) is used for the transmission path from the base station to the wireless device. The expression uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
A heterogeneous network (hetnet) is a network with more than one cell type and/or base station type. The difference between the different cell and/or base station types may relate to several different features, but most often there is a difference between the maximum transmit power (i.e. the power amplifiers differ) which also relates to the cell size. A typical hetnet scenario comprises a higher power cell and a lower power cell, which may be referred to as macro cell and PICO cell respectively, where the macro cell is served by a base station transmitting with higher power than a base station serving the PICO cell. The purpose of PICO cells in general is often to offload macro cells and increase the capacity per area unit. The downlink power from the macro cell is typically much higher than from the PICO cell (e.g. 20 W to 2 W) and the macro cell will cover a larger area in the downlink compared to the PICO cell. When a downlink connection is equally good from the macro and the PICO cell, the PICO uplink connection may be much better, e.g. 20/2=10 times better in case of the example of a 20 W vs. 2 W power difference. In heterogeneous networks there may thus be locations where a wireless device has the best downlink connection when served in a macro cell, since the macro cell transmits with higher power, but at the same time a better uplink connection when served in the PICO cell, since the wireless device transmits with same power to both the macro and PICO cell but may be located closer to the PICO cell. Such lack of compliance between downlink and uplink connections is related to several problems.
A first problem is when the reason for adding the PICO cell is to offload the macro cell as much as possible, i.e. it is desirable that wireless devices are served in the PICO cell. This is however counteracted to some extent by the higher downlink transmitting power from the macro cell since a wireless device will receive a relatively higher signal strength from the macro cell, which typically is what determines the choice of serving cell and base station.
A second problem is that a wireless device that is closer to the PICO cell than to the maro cell may be served in the macro cell even though there is actually a better UL connection to the PICO cell. As a consequence, such a wireless device will experience worse link performance than if it had been served by the PICO cell, at least in the uplink.
A third problem is that a wireless device that is served in the macro cell and located in the outskirt of the macro cell, close to a border of the PICO cell, will transmit with relatively high power in the uplink to the macro cell and thereby cause relatively high interference to communications in the closer PICO cell. This has negative impact on the uplink performance of any wireless device being served in the PICO cell, which e.g. may be identified by studying measurement graphs from interference measurements of neighbouring cells.